(a) Field of the Invention
The present invention relates to a polymerization method for aqueous acrylic latex, and more particularly to a polymerization method for acrylic latex without emulsifier which enables providing a water dispersible acrylic latex that is environmentally friendly and non-polluting, in which particle diameters of the resin latex are small and uniformly distributed with excellent outward appearance; its specific gravity is slightly greater than water, and has low acid value, superior storage stability, with good permeability that facilitates construction. Moreover, water resisting property as a dry film coating is good, tensile strength is excellent, and provides superior abrasion performance.
(b) Description of the Prior Art
Traditionally, preparation of acrylic latex comprises acrylic acid monomers in an aqueous solution, upon which emulsification polymerization reaction is carried out using low molecular weight emulsifiers, and under the effect of emulsifiers, mechanical stirring and an initiator causes the monomers in the water or other solvent to disperse and form an emulsion for free radical polymerization to be carried out thereon. However, if manufacture of a solid polymer is required, a multiple step procedure involving emulsion breaking, washing, dehydration, drying must be carried out, resulting in high production cost. Moreover, impurities, such as emulsifiers, in the obtained polymer product are difficult to clean out, and affect electrical properties, and thus unsuitable for use.
When drying the obtained emulsion, the emulsifiers easily exude out from within the polymer, thereby contaminating the surface of the acrylic latex and reducing the bonding strength thereof between other materials, and thus not suitable for use.
In order to solve the problem brought about by participation of the emulsifier in the emulsification polymerization reaction, patent document 1 (Japanese Patent Publication No. 2006-206907) describes using non-ionic surface active agents, including polyethylene glycol octyl phenyl ether, polyethylene glycol nonyl phenyl ether, and polyethylene glycol decyl phenyl ether, to synthesize acrylic latex. And although facilitating in preventing environmental pollution, however, such environmental hormone compounds are harmful to the human body, and thus limits use thereof.
In general, when carrying out emulsification polymerization, solubility of the monomers in aqueous phase is relatively small, thus the monomer droplets do not easily migrate to micelles, and organic solvents such as acetone or isopropyl alcohol must be added to assist solubilization. Some people have tried adding co-emulsifiers, and stirring at high speed to cause the monomers to disperse in the water and become micron sized monomer droplets to serve as the primary polymerization site. Such so-called fine emulsion polymerization is used to solve the problem of the difficulty in dissolving the monomers.
The aforementioned fine emulsion polymerization method provides a relatively good solution to the problem of the difficulty in dissolving the monomers, while some people use solvent-soluble initiators to initiate direct polymerization of the monomers in the monomer droplets. The process of the monomer droplets migrating to micelles is not necessary, and the droplets are able to directly form cores, thereby avoiding the problem of indissolubility of the monomers.
Some people use a compound emulsifier composed of ionic surface active agents and long chain aliphatic alcohols or long chain alkanes to cause monomers in a fine emulsion to form stable nanometer droplets (30˜500 nm). Because the ionic surface active agents and the compound emulsifiers produce osmotic pressure that neutralizes the pressure difference between large and small monomer droplets, and reduces monomer diffusion between different sized droplets, thus, stability of the small droplets is greatly increased, thereby enabling the fine emulsion to obtain sufficient dynamic stability. Accordingly, water and solvent, surface active agents and adjuvants are able to spontaneously form thermodynamically stable emulsion.
However, during the polymerization process, variations in the water, solvent, emulsifier and adjuvant emulsifier of the system composition, or polymerization reaction temperature, hydrophilic or oleophilic groups can cause the surface active agent molecules in the solution and the produced micelles to mutually transform into three types of structures including oil in water O/W, water in oil W/O and bicontinue B.C, and form polymer microemulsion with small average particle diameter, low surface tension, greatly strengthened wettability and permeability and better stability.
Patent documentation 2 (Japanese Patent Publication No. 07-331208) points out that in the presence of a polymeric emulsifier composed of reactive emulsifiers (A), and styrenes and/or alkyl (meth)acrylates (B) and acrylic acid and/or methacrylic acid anionic monomers (C) as the primary constituents, emulsifying tackifier resin enables obtaining tackifier resin latex; and polymer emulsion containing acrylic esters and/or latex for adhesive use enables obtaining aqueous adhesive constituents.
However, the aforementioned reactive emulsifier (A) refers to use proportion of carbon-carbon double bonded surface active agents having hydrophilic groups and hydrophobic groups in-molecule reaching as high as 10˜50 wt % of the composed monomer composition. Moreover, the tackifier resin must participate in the reaction.
Hence, the trend for avoiding the problems brought about participation of the emulsifier in the reaction lies in the emulsion polymerization process adding no emulsifier whatsoever or only the addition of a trace amount of emulsifier (concentration of which is less than the critical micelle concentration CMC) during the reaction process or a portion of the obtained polymers being provided with polymeric emulsifier characteristics. This being non-soap emulsion polymerization without the addition of conventional emulsifiers, enabling obtaining a clean surface, and monodispersed latex particles, while at the same time avoiding the use of emulsifiers which affect the environment.
In addition, patent documentation 1 points out that when synthesizing acrylic latex, water-soluble tackifying resin is used without the use of emulsifiers to carry out emulsion polymerization, thereby enabling the freezing stability of acrylic latex to be increased, as well as enabling reuse when thawed after freezing, thus completely satisfying such physical properties using a one-time synthesis. Although achieving the effectiveness of shortening the procedure, however, because the tackifying resin used is water-soluble, the obtained latex film is deficient in water resistance, and thus not suitable for coating purposes.
In order to solve the problem of water resistance, measures should be taken regarding stability of the latex colloidal particles related to the amount of electrolyte and other low molecules contained in the latex film. The stability of the latex colloidal particles is primarily determined by surface-active oligomers, and the larger the surface charge concentration of the latex colloidal particles, the better the stability. Surface stability of the colloidal particles is still determined by hydrophilicity of the particle surfaces of the latex colloidal particles, and the greater the hydrophilicity, the better the stability. Accordingly, because polarity of the latex colloidal particles causes interaction between the particle surfaces and the aqueous phase interface and reduces Gibbs free energy at the interface, thus, an increase in particle stability of the latex colloidal particles results.
Hence, the issues of the present invention urgently awaiting to be solved are avoiding the problems of contamination brought about by using non-ionic surface active agents and coemulsifiers resolving the difficulty in dissolving monomers, and implements non-soap emulsion polymerization without the addition of conventional emulsifiers, which increases stability of small droplets during emulsification, while surface active oligomers increase the stability of the latex colloidal particles. Moreover, latex monodispersed particles are obtained with clean surfaces, while at the same time avoiding the use of emulsifiers which affect the environment,